The next thing____ must be done is to make a plan.这个定语从句中 必须用that么 which可以么

问题描述:

The next thing____ must be done is to make a plan.这个定语从句中 必须用that么 which可以么
先行词是thing 必须用that么?
先行词被the next 修饰了,必须用that么?

先行词有the next 修饰,用that